How would I build a twisted water slide?
Using the Add Curve:Extra Objects shipped add-on, create a Spiral, Archimedes, 2 anticlockwise turns, type: Poly. Cut off the ends appropriately, and E extrude the new ends to make the straight sections.
Adjust the native Full bevel of the curve in its data tab, for depth and resolution. (object data tab > Shape and Geometry panels. Shape: 3D, Fill>Full.. and Geometry: Bevel>Depth and Bevel>Resolution)
In Object Mode AltC convert to mesh, then assign and apply a Solidify modifier
K with C and Z make a cut through the pipe, constrained to 45 degrees. Delete surplus faces and CtrlE > Bridge Edge Loops to repair the rim. Find or create a few edge loops round the outside skin, and Ctrl B bevel them to the width of the joints. With the new face rings selected, E extrude, right-click to cancel transform, AltS scale the extrusion along its normals.


Then you can add a Bevel modifier by angle (segments 2, profile 1.0) to sharpen edges, and add a Subdivision Surface modifier.. unless you just want to stick with low-poly, and Auto Smooth by angle in the slide's data panel.